The seed of the development of modern solid-state semiconductor electronics goes back to the 1930s when it was realized that some solid-state semiconductors and their junctions offer the possibility of controlling the number and the direction of flow of charge carriers through them.
Simple excitations like light, heat, or small applied voltage can change the number of mobile charges in a semiconductor.
Note that the supply and flow of charge carriers in the semiconductor devices are within the solid itself, while in the earlier vacuum tubes/valves, the mobile electrons were obtained from a heated cathode and they were made to flow in an evacuated space or vacuum.
No external heating or large evacuated space is required by the semiconductor devices.
They are small in size, consume low power, operate at low voltages, and have a long life and high reliability.
Even the Cathode Ray Tubes (CRT) used in television and computer monitors which work on the principle of vacuum tubes are being replaced by Liquid Crystal Display (LCD) monitors with supporting solid-state electronics.
Much before the full implications of the semiconductor devices were formally understood, a naturally occurring crystal of galena (lead sulphide, PBS) with a metal point contact attached to it was used as a detector of radio waves.
On the basis of the relative values of electrical conductivity (σ) or resistivity ( ρ = 1/σ ), the solids are broadly classified as:
Metals: They possess very low resistivity (or high conductivity). ρ~10–2–10–8Ωm;σ~102–108Sm–1
Semiconductors: They have resistivity or conductivity intermediate to metals and insulators. ρ~10–5–106Ωm;σ~105–10–6Sm–1
Insulators: They have high resistivity (or low conductivity). ρ~1011–1019Ωm;σ~10–11–10–19Sm–1
The values of ρ and σ have given above are indicative of the magnitude and could well go outside the ranges as well.
Relative values of the resistivity are not the only criteria for distinguishing metals, insulators and semiconductors from each other. There are some other differences too.
Semiconductors could be:
Elemental semiconductors: Si and Ge
Compound semiconductors: Examples are:
Inorganic: CdS, GaAs, CdSe, InP, etc.
Organic: anthracene, doped phthalocyanines, etc.
Organic polymers: polypyrrole, polyaniline, polythiophene, etc.
Most of the currently available semiconductor devices are based on elemental semiconductors Silicon (Si) or Germanium (Ge) and compound inorganic semiconductors.
Some semiconductor devices are junction diodes (a 2-electrode device) and bipolar junction transistor (a 3-electrode device).
However, after 1990, a few semiconductor devices using organic semiconductors and semiconducting polymers have been developed signaling the birth of a futuristic technology of polymer-electronics and molecular-electronics.